Procedure for the dynamic diagnosis of an exhaust gas probe

ABSTRACT

The invention concerns a procedure for the dynamic diagnosis of an exhaust gas probe disposed in an exhaust gas duct of an internal combustion engine after the emission control system. Provision is made according to the invention for the dynamic diagnosis to be implemented simultaneously with a precipitous change of a Lambda value of the exhaust gas from rich to lean or from lean to rich. During the operating time of an internal combustion engine, step changes in the mixture composition occur for different reasons from rich to lean and from lean to rich. If a dynamic diagnosis of the exhaust gas probe is implemented during such a step change of the mixture composition, provision must not be specially made for such a step change in the diagnosis, which can lead to increased emissions. It is simply to be taken into regard that the mixture composition is suited to the effect that the diffusion barrier is not eliminated due to mixture displacements, which are too large.

BRIEF DESCRIPTION OF THE INVENTION

The invention concerns a procedure for the dynamic diagnosis of an exhaust gas probe disposed behind the emission control system in the exhaust gas duct of an internal combustion engine.

The storage capability for oxygen of an emission control system is utilized for the purpose of accumulating oxygen in the lean phases and giving it off in the rich phases. The ability to convert oxidable toxic gas components is thereby achieved. An exhaust gas probe located downstream from the emission control system serves then to monitor the oxygen storage capability of the emission control system. The oxygen storage capability must be monitored within the framework of the On-Board-Diagnosis because it represents a measurement for the conversion capability of the emission control system. In order to determine the oxygen storage capability, the emission control system is either initially filled with oxygen in a lean phase, and in a rich phase is subsequently emptied of an exhaust gas of a known Lambda while taking into consideration the amount of exhaust gas which has passed through the emission control system; or the emission control system is initially emptied of oxygen in a rich phase and subsequently in a lean phase is filled with an exhaust gas of a known Lambda taking into consideration the amount of exhaust gas which has passed through the emission control system. The lean phase is terminated if the exhaust gas probe downstream from the emission control system detects the oxygen, which can no longer be stored by the emission control system. Likewise a rich phase is terminated if the exhaust gas probe detects the passage of rich exhaust gas. Furthermore, an output signal of the exhaust gas probe serves as additional information for a closed-loop Lambda control, which, however, to a great extent relies on the output signal of a Lambda probe disposed before the emission control system.

If the exhaust gas probe ages, the output signal of the exhaust gas probe reacts more slowly to changes in the exhaust gas composition, and deviations in the diagnosis or the emission control system can occur. These deviations can lead to the point, where an emission control system, which is no longer correctly working, is wrongly evaluated to be operative. A known procedure for the diagnosis of an emission control system evaluates the ratio of the amplitudes of the output signals of the Lambda probe disposed before emission control system to those of the exhaust gas probe downstream from the emission control system. An operative emission control system dampens by means of its storage capability the amplitude of an oscillation of the oxygen content of the exhaust gas at the outlet of the internal combustion engine, so that the ratio of the amplitudes before and after the emission control system result in a high value. A delayed reaction of the exhaust gas probe downstream from the emission control system leads, however, likewise to a reduction of the amplitude of its output signal, whereby the oxygen storage capability of the emission control system is assessed as being too high. An emission control system no longer corresponding to the demands can, thus, under certain circumstances wrongly be classified as being in correct working order.

A dynamic diagnosis is complicated due to the fact that the output signal of the exhaust gas probe is dependent on the beginning and final Lambda value in the case of a rich-lean or lean-rich step change. Moreover the influence of the emission control system described above factors in, which is affected further by the influences of temperature and age on the emission control system.

A procedure for the dynamic diagnosis of an exhaust gas probe is put forth in the German patent DE 19722334. The exhaust gas probe is disposed in the exhaust gas behind the emission control system. The rate of change of an output signal of the exhaust gas probe is used as an assessment criterion, which, for example, occurs after the beginning of a phase in the coasting (overrun) mode. It is a disadvantage in this case that this procedure works only when a very large air mass flow (>>50 kg/h) is present, as only then the effect of the catalytic converter can be disregarded. In such operating states, undesirable conditions can, however, emerge when resetting after the coasting (overrun) phase.

It is the task of the invention to provide a procedure for the dynamic diagnosis of an exhaust gas probe disposed behind an emission control system, which allows for a reliable evaluation.

SUMMARY OF THE INVENTION

The task is thereby solved, in that the dynamic diagnosis is implemented simultaneously with a precipitous change of the Lambda value of the exhaust gas from rich to lean or from lean to rich. During the operating time of an internal combustion engine, step changes of the mixture composition from rich to lean and from lean to rich occur for different reasons. If the dynamic diagnosis of the exhaust gas probe is implemented during one such step change of the mixture composition, provision does not have to be specially made for such a step change in the diagnosis, which can lead to increased emissions. It is simply important to pay attention that the mixture composition is sufficiently suited to prevent the diffusion barrier from being disabled by mixture displacements, which are too large.

If the dynamic diagnosis is simultaneously implemented with a diagnosis of the emission control system with a precipitous change of a Lambda value, the fact that the state of the emission control system is known can be advantageously utilized and perturbations can be eliminated as far as possible. Moreover, it is advantageous that no additional emissions are generated, as would be the case when implementing a rich phase, which is specially conducted for the diagnosis of the exhaust gas probe with a passage of rich exhaust gas through the emission control system.

If the dynamic diagnosis is implemented after a change of the exhaust gas composition from a Lambda smaller than 1 to a Lambda greater than 1, the dynamic diagnosis to determine a step function response to an output signal of the probe during a step change from rich to lean can be implemented after a rich conditioning of the emission control system during the subsequent rich-lean step change.

If a rich conditioning of the emission control system and the precipitous change immediately following of the exhaust gas composition from a Lambda smaller than 1 to a Lambda greater than 1 are used to determine a response rate diagnosis from rich to lean, the response rate diagnosis can be implemented without provision being made for a special alteration of the exhaust gas composition within the scope of the diagnosis of the emission control system.

If the dynamic diagnosis is implemented after a change of the exhaust gas composition from a Lambda greater than 1 to a Lambda smaller than 1, a dynamic diagnosis can be implemented after a cat gutting and the lean-rich step change, which occurs in the process, to determine the step function response from lean to rich.

If a gutting of the emission control system and the lean-rich step change, which occurs in the process, are used for the determination of a step function response from lean to rich, an emission of toxic pollutants can be reduced, as the response diagnosis can be simultaneously implemented with the diagnosis of the emission control system.

If the dynamic diagnosis is discontinued when the diagnosis of the emission control system recognizes it to be in good working order and the diagnosis of the emission control system is completed ahead of time, an unnecessary emission of exhaust gas can be avoided. In so doing, the principle is utilized that in the case of emission control systems with a sufficient conversion capability, the dynamics of the exhaust gas probe located downstream from the emission control system are not relevant.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail below using an example of embodiment depicted in the figures. The following are shown:

FIG. 1 a technical layout in schematic representation, in which the procedure according to the invention can be applied,

FIG. 2 a probe output signal of the exhaust gas probe during a dynamic diagnosis

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the technical layout, in which the procedure according to the invention can be deployed for the dynamic diagnosis of an exhaust gas probe 17. Air is supplied to an internal combustion engine 10 by way of an air feed 11, and its mass is determined with an air mass meter 12. The air mass meter 12 can be embodied as a hot film air mass meter. The exhaust gas of the internal combustion engine 10 is discharged via an exhaust gas duct 18, whereby provision is made for an emission control system 16 behind the internal combustion engine 10 in the direction of flow of the exhaust gas. Provision is made for an engine management system 14 to control the internal combustion engine 10. The engine management system 14 delivers on the one hand fuel to the internal combustion engine 10 by way of a fuel metering 13 and on the other hand is provided with signals from the air mass meter 12 and a Lambda probe 15 disposed in the exhaust gas duct 18 as well as from an exhaust gas probe 17 disposed in the exhaust gas discharge pipe 18. The Lambda probe 15 determines a Lambda actual value of a fuel-air-mixture delivered to the internal combustion engine 10. The Lambda probe 15 can be embodied as a wide band Lambda probe. The exhaust gas probe 17 determines the exhaust gas composition after the emission control system 16. The exhaust gas probe 17 can be designed as a step change probe.

A time sequence diagram 20 in FIG. 2 shows a progression of a probe output signal 21 of the exhaust gas probe 17 in operating cases, which are suited to a dynamic diagnosis in accordance with the procedure according to the invention. All of the signals and operating phases are plotted along a time axis 25. Within the scope of a diagnosis of the emission control system, a rich conditioning 22 is conducted, in which the oxygen present in the emission control system is removed and in which the probe output signal 21 increases. At the end of a subsequent lean phase 23, lean exhaust gas is discharged after the emission control system, and a step change to a smaller voltage occurs in the probe output signal. This step change can in accordance with the invention be utilized in the dynamic diagnosis. At the end of a subsequent gutting phase 24, for which provision has been made and in which the emission control system is emptied, the probe output signal 21 increases precipitously. This step change can also in accordance with the invention be used in the dynamic diagnosis. 

1. A method of diagnosing an exhaust gas probe disposed in an exhaust gas duct of an internal combustion engine, the method comprising dynamically diagnosing the exhaust gas probe simultaneously with a precipitous change of a Lambda value of an exhaust gas from rich to lean or from lean to rich.
 2. A method according to claim 1, wherein dynamically diagnosing includes dynamicaly diagnosing simultaneously with a diagnosis of an emission control system and with a precipitous change of the Lambda value of the exhaust gas.
 3. A procedure according to claim 2, wherein dynamically diagnosing includes diagnosing after a change of an exhaust gas composition from a Lambda smaller than 1 to a Lambda greater than
 1. 4. A method according to claim 1, further comprising determining a step function response from rich to lean based on a rich conditioning of an emission control system and the immediately subsequent precipitous change of an exhaust gas composition from a Lambda smaller than 1 to a Lambda greater than
 1. 5. A method according to claim 2, wherein dynamically diagnosing is implemented after a change of an exhaust gas composition from a Lambda greater than 1 to a Lambda smaller than
 1. 6. A method according to claim 1, further comprising determining a response diagnosis from lean to rich from a gutting of an emission control system and a lean-rich step change, which occurs in the process.
 7. A method according to claim 1, wherein dynamically diagnosing is discontinued, if a diagnosis of an emission control system recognizes it to be in good working order, and the diagnosis of the emission control system is completed ahead of time. 